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| United States Patent |
5,198,041
|
|
Takemoto
, et al.
|
March 30, 1993
|
Shape memory stainless steel excellent in stress corrosion cracking
resistance and method thereof
Abstract
A shape memory stainless steel containing more than 10% by weight of Cr
excellent in resistance to stress corrosion cracking and having sufficient
function as a shape memory alloy, which comprises, by weight, up to 0.10%
of C, 3.0 to 6.0% of Si, 6.0 to 25.0% of Mn, up to 7.0% of Ni, more than
10.0% and not more than 17.0% of Cr, 0.02 to 0.3% of N, 2.0 to 10.0% of Co
and more than 0.2% and not more than 3.5% of Cu, and at least one selected
from up to 2.0% of Mo, 0.05 to 0.8% of Nb, 0.05 to 0.8% of V, 0.05 to 0.8%
of Zr, 0.05 or 0.8% of Ti, the balance being Fe and unavoidable
impurities, the alloying components being adjusted so that a D value is
not less than-26.0, wherein the D value is defined by the following
equation:
D=Ni+0.30.times.Mn+56.8.times.C+19.0.times.N+0.73.times.Co+Cu
-1.85.times.[Cr+1.6.times.Si+Mo+1.5.times.(Nb+V+Zr+Ti)].
| Inventors:
|
Takemoto; Toshihiko (Tokuyama, JP);
Kinugasa; Masayuki (Hiroshima, JP);
Tanaka; Teruo (Kure, JP);
Igawa; Takashi (Yamaguchi, JP)
|
| Assignee:
|
Nisshin Steel Co., Ltd. (Tokyo, JP)
|
| Appl. No.:
|
835433 |
| Filed:
|
February 25, 1992 |
| PCT Filed:
|
August 4, 1990
|
| PCT NO:
|
PCT/JP90/01001
|
| 371 Date:
|
February 25, 1992
|
| 102(e) Date:
|
February 25, 1992
|
| PCT PUB.NO.:
|
WO91/02827 |
| PCT PUB. Date:
|
March 7, 1991 |
Foreign Application Priority Data
| Current U.S. Class: |
148/402; 148/563 |
| Intern'l Class: |
C21D 008/00; C22C 038/38 |
| Field of Search: |
148/402,563
|
References Cited
U.S. Patent Documents
| 4933027 | Jun., 1990 | Moriya et al. | 148/402.
|
| 5032195 | Jul., 1991 | Shin et al. | 148/402.
|
| Foreign Patent Documents |
| 62-170457 | Jul., 1987 | JP.
| |
| 63-216946 | Sep., 1988 | JP.
| |
Primary Examiner: Yee; Deborah
Attorney, Agent or Firm: Lowe, Price, LeBlanc & Becker
Claims
What is claimed is:
1. A shape memory stainless steel excellent in resistance to stress
corrosion cracking, which comprises, by weight, up to 0.10% of C, 3.0 to
6.0% of Si, 6.0 to 25.0% of Mn, up to 7.0% of Ni, more than 10.0% and not
more than 17.0% of Cr, 0.02 to 0.3% of N, 2.0to 10.0% of Co and more than
0.2% and not more than 3.5% of Cu, the balance being Fe and unavoidable
impurities, the alloying components being adjusted so that a D value is
not less than -26.0, wherein the D value is defined by the following
equation:
D=Ni+0.30.times.Mn+56.8.times.C+19.0.times.N+0.73.times.Co+Cu-1.85.times.
(Cr+1.6.times.Si).
2. A shape memory stainless steel excellent in resistance to stress
corrosion cracking, which comprises, by weight, up to 0.10% of C, 3.0 to
6.0% of Si, 6.0 to 25.0% of Mn, up to 7.0% of Ni, more than 10.0% and not
more than 17.0% of Cr, 0.02 to 0.3% of N, 2.0 to 10.0% of Co and more than
0.2% and not more than 3.5% of Cu, and at least one selected from up to
2.0% of Mo, 0.05 to 0.8% of Nb, 0.05 to 0.8% of V, 0.05 to 0.8% of Zr,
0.05 to 0.8% of Ti, the balance being Fe and unavoidable impurities, the
alloying components being adjusted so that a D value is not less than
-26.0, wherein the D value is defined by the following equation:
D=Ni+0.30.times.Mn+56.8.times.C+19.0.times.N+0.73.times.Co+Cu-1.85.times.
[Cr+1.6.times.Si+Mo+1.5.times.(Nb+V+Zr+Ti)].
3. A method of shape memorizing and shape recovering of a stainless steel
excellent in resistance to stress corrosion cracking, which comprises the
steps of:
processing a stainless steel to an article of a predetermined shape and
annealing the article to memorize the shape, said steel comprising, by
weight, up to 0.10% of C, 3.0 to 6.0% of Si, 6.0 to 25.0% of Mn, up to
7.0% of Ni, more than 10.0% and not more than 17.0% of Cr, 0.02 to 0.3% of
N, 2.0 to 10.0% of Co and more than 0.2% and not more than 3.5% of Cu, and
optionally at least one selected from up to 2.0% of Mo, 0.05 to 0.8% of
Nb, 0.05 to 0.8% of V, 0.05 to 0.8% of Zr, 0.05 to 0.8% of Ti, the balance
being Fe and unavoidable impurities, the alloying components being
adjusted so that a D value is not less than -26.0, wherein the D value is
defined by the following equation:
D=Ni+0.30.times.Mn+56.8.times.C+19.0.times.N+0.73.times.Co+Cu-1.85.times.
[Cr+1.6.times.Si+Mo+1.5.times.(Nb+V+Zr+Ti)],
deforming the annealed article at a temperature of not higher than room
temperature, and
heating the deformed article to a temperature of at least 100.degree. C.
and allowing it to cool to room temperature, thereby to recover the
memorized shape.
4. A method of shape memorizing and shape recovering of a stainless steel
excellent in resistance to stress corrosion cracking, which comprises the
steps of:
processing a stainless steel to an article of a predetermined shape and
annealing the article, said steel comprising, by weight, up to 0.10% of C,
3.0 to 6.0% of Si, 6.0 to 25.0% of Mn, up to 7.0% of Ni, more than 10.0%
and not more than 17.0% of Cr, 0.02 to 0.3% of N, 2.0 to 10.0% of Co and
more than 0.2% and not more than 3.5% of Cu, and optionally at least one
selected from up to 2.0% of Mo, 0.05 to 0.8% of Nb, 0.05 to 0.8% of V,
0.05 to 0.8% of Zr, 0.05 to 0.8% of Ti, the balance being Fe and
unavoidable impurities, the alloying components being adjusted so that a D
value is not less than -26.0, wherein the D value is defined by the
following equation:
D=Ni+0.30.times.Mn+56.8.times.C+19.0.times.N+0.73.times.Co+Cu-1.85.times.
[Cr+1.6.times.Si+Mo+1.5.times.(Nb+V+Zr+Ti)],
subjecting the article one or more times to a training cycle comprising
deformation at a temperature of not higher than room temperature and
heating to a temperature of from 450.degree. C. and 700.degree. C., and
allowing the so-trained article to cool to room temperature, thereby to
achieve and memorize a primary shape,
deforming the primary shape memorized article to a desired secondary shape
at a temperature of not higher than room temperature, heating it to a
temperature of at least 100.degree. C. and allowing it to cool to room
temperature, thereby to recover the primary shape.
Description
FIELD OF THE INVENTION
The invention relates to a shape memory stainless steel excellent in shape
memory effect and a method for enhancing shape memory effect thereof. More
particularly, the invention relates to a shape memory stainless steel
excellent in resistance to stress corrosion cracking which can
advantageously develop its shape memory effect when used as fixing or
fastening parts of machines, or as a pipe joint.
BACKGROUND OF THE INVENTION
As alloys exhibiting shape memory effect, there are known nonferrous-metal
alloys including Ni--Ti alloys and Cu alloys as well as ferrous metal
alloys such as Fe--Pd alloys, Fe--Ni alloys and Fe--Mn alloys. Among
others, Fe--Mn alloys are inexpensive, and thus, because of their
commercial value, various alloys of this Fe--Mn series are reported in
patent literatures, for example, Fe--(15.9-30.0%) Mn alloys in JP A
55-73846, Fe--Mn--(Si, Ni, Cr) alloys in JP A 55-76043,
Fe--(20-40%)Mn--(3.5-8%)Si alloys in JP A 61-76647 and Fe--(15-30%)Mn--N
alloys in JP A 63-216946. furthermore, JP A 62-112720 discloses a method
for enhancing shape memory effect of a Fe--Mn--Si alloy wherein a
so-called training effect by repeating a cycle of working at a rate of up
to 20% and heating to a temperature of at least 400.degree. C. is
utilized.
However, ferrous metal shape memory alloys are generally disadvantageous in
low corrosion resistance. JP A 61-201761 discloses examples of Fe--Mn--Si
alloys whose corrosion resistance is improved by adding Cr. However, the
Cr content taught is too low, i.e. not more than 10.0%, to achieve
corrosion resistance well comparable with that of stainless steels.
Furthermore, JP A 63-216946 teaches to improve corrosion resistance of
ferrous metal shape memory alloys by adding Cr. Again, however, the Cr
content disclosed is 10% or less and it is not taught how to realize a
desired level of shape memory characteristics with the ferrous metal shape
memory alloys having Cr, which is a ferrite former, in excess of 10%
incorporated therein.
On the other hand, as to general stainless steels, "Scripta Metallurgica,
1977, vol. 5, pp.663.about.667" reports that SUS304 steel exhibits shape
memory effect, if it is deformed at -196.degree. C. and then heated to
room temperature, however, its shape recovery is too small to put it to
practical use.
OBJECT OF THE INVENTION
An object of the invention is to provide a shape memory alloy containing
more than 10% of Cr, which alloy is capable of exhibiting such a shape
memory effect that even though the temperature of secondary deformation is
not very low, for example, even though the temperature of secondary
deformation is slightly below room temperature, when it is heated to
moderately elevated temperature after the secondary deformation, it can
recover its primary shape prior to the secondary deformation, and which
alloy does not substantially suffer from stress corrosion cracking that
may be a problem when the alloy is used as a pipe joint or the like.
DISCLOSURE OF THE INVENTION
According to the invention, there is provided a shape memory stainless
steel excellent in resistance to stress corrosion cracking, which
comprises, by weight, up to 0.10% of C, 3.0 to 6.0% of Si, 6.0 to 25.0% of
Mn, up to 7.0% of Ni, more than 10.0% and not more than 17.0% of Cr, 0.02
to 0.3% of N, 2.0 to 10.0% of Co and more than 0.2% and not more than 3.5%
of Cu, and optionally at least one selected from up to 2.0% of Mo, 0.05 to
0.8% of Nb, 0.05 to 0.8% of V, 0.05 to 0.8% of Zr, 0.05 to 0.8% of Ti, the
balance being Fe and unavoidable impurities, the alloying components being
adjusted so that a D value is not less than -26.0, wherein the D value is
defined by the following equation:
D=Ni+0.30.times.Mn+56.8.times.C+19.0.times.N+0.73.times.Co+Cu-1.85.times.[C
r+1.6.times.Si+Mo+1.5.times.(Nb+V+Zr+Ti)].
When the steel having the above defined chemical composition is processed
to an article of a predetermined shape, annealed to memorize the shape,
deformed at a temperature of not higher than room temperature, heated to a
temperature of at least 100.degree. C. and allowed to cool to room
temperature, the memorized shape can be recovered at a high percent of
recovery. The processing temperature prior to the annealing may room
temperature or higher. The article may be in the form of plates, pipes or
any other arbitrary shapes. While the article may be deformed at room
temperature, for example, about at 20.degree. C., the lower the
deformation temperature the higher percent of shape recovery can be
achieved. The deformation may be done, as with conventional shape memory
alloys, by drawing, pulling, compression or bending, or by diameter
expansion of tubular articles.
If the steel having the above defined chemical composition is processed to
an article of a predetermined shape, annealed, subjected one or more times
to a training cycle comprising deformation to at a temperature of not
higher than room temperature (primary deformation) and heating to a
temperature of from 450.degree. C. and 700.degree. C., allowed to cool to
room temperature, thereby to achieve and memorize a primary shape,
deformed to a desired secondary shape at a temperature of not higher than
room temperature (secondary deformation), heated to a temperature of at
least 100.degree. C. and allowed to cool to room temperature, the primary
shape can be recovered at a still higher percent of recovery.
The stainless steel according to the invention has excellent resistance to
stress corrosion cracking in addition to general corrosion resistance
inherent to stainless steels.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a perspective view of a test piece in the constrained condition
which was subjected to the stress corrosion cracking test noted below.
Under this condition the test piece is prevented from recovering its
shape, that is it has a residual stress.
DETAILED DESCRIPTION OF THE INVENTION
In order to achieve the objects, we have extensively studied influences of
alloying components as well as mechanical working and heat treating
conditions on shape memory effect of corrosion resistive Fe--Cr steels. As
a result, we have found that if a Cr--Fe based metal having more than 10%
of Cr is incorporated with appropriate amounts of Mn, Si and Co and the
contents of C, N and Ni are properly controlled, the metal may exhibit a
single austenitic phase in the annealed condition with no .delta.-ferritic
and martensitic phases. We have also found that even if such a metal is
deformed at a temperature not higher than room temperature, formation of
permanent strain of work induced martensite (.alpha.') and dislocation can
be suppressed, and in particular, when the metal is deformed at a
temperature of 0.degree. C. or lower, formation of work induced
.epsilon.-phase can be facilitated and in consequence, after deformation,
if the metal is heated to its As point (temperature at which
.epsilon.-phase starts to transform to .gamma.-phase) or higher, the metal
exhibits excellent shape memory effect. We have further found that the
shape memory effect will be remarkably enhanced by carrying out one or
more times a training treatment comprising deformation at a temperature of
not higher than room temperature and heating at a temperature of
450.degree. C. or higher.
Such a shape memory stainless steel has a high general corrosion resistance
well comparable with other stainless steels. However, in some
applications, for example, when used as a pipe joint, since the steel
which has shape-recovered under constraint has an internal strain
(residual stress), resistance to stress corrosion cracking is of
importance. General information about resistance to stress corrosion
cracking of general stainless steels, such as SUS304, is not necessarily
applicable to shape memory stainless steels of high Mn-high Si-high Co
series. On such Fe--Cr shape memory stainless steels incorporated with
appropriate amounts of Mn, Si and Co and having properly controlled C, N
and Ni contents. As a result, we have found that while C, Mn and Ni
adversely affect resistance to stress corrosion cracking, Co, N and Cu, in
particular N and Cu, enhance resistance to stress corrosion cracking. We
have further found that Cu is also effective to enhance shape memory
effect.
Reasons for the restrictions of the alloying components of the stainless
steel alloy used herein will now be described.
C is a strong austenite former and serves effectively formation of a
.delta.-ferritic phase in the annealed condition. Further C is a useful
element to improve shape memory effect. However, C adversely affects
resistance to stress corrosion cracking. Moreover, if C is included so
much, when a training cycle of deformation in the temperature range of not
higher than room temperature and heating in the temperature range of at
least 450.degree. C. is carried out one or more times, Cr carbide is
produced to disadvantageously deteriorate corrosion resistance and
workability. For this reason the content of C must be up to 0.10%.
Since during the step of deformation Si acts to prevent generation of
permanent strain and to facilitate formation of a work induced
.epsilon.-phase, Si is indispensable to develop excellent shape memory
effect in the steel according to the invention and, thus, at least 3.0% of
Si must be included. However, Si is a strong ferrite former, and
therefore, the presence of an excessive amount of Si, not only retains so
much .delta.-ferritic phase in the annealed condition to deteriorate shape
memory effect, but also adversely affects hot workability of the steel to
make the steel making difficult. Accordingly, the upper limit for Si is
now set as 6.0%.
Mn is an austenite former and serves to control formation of a
.delta.-ferrite phase in the annealed condition. Further since during the
step of deformation Mn acts to prevent generation of permanent strain and
to facilitate formation of a work induced .epsilon.-phase, Mn is effective
to enhance shape memory effect. For these purposes at least 6.0% of Mn is
required. However, Mn adversely affects resistance to stress corrosion
cracking, and if Mn is included so much, on the contrary, it restricts
formation of a work induced .epsilon.-phase to decrease shape memory
effect, and therefore, the upper limit for Mn is now set as 25.0%.
Ni is an austenite former and is useful to prevent formation of a
.delta.-ferrite phase in the annealed condition. However, if Ni is
included so much, permanent strain may occur in the step of deformation at
low a temperature to decrease shape memory effect and lowers resistance to
stress corrosion cracking. Accordingly, the upper limit for Ni is now set
as 7.0%.
Cr is an indispensable element for stainless steels and more than 10% of Cr
is required to achieve general high corrosion resistance. Further since Cr
restricts generation of permanent strain during the step of deformation at
a low temperature, Cr is effective to improve shape memory effect.
However, since Cr is a ferrite former, if it is included so much, a
.delta.-ferrite phase is likely to remain in the annealed condition,
thereby adversely affecting shape memory effect. Accordingly, the upper
limit for Cr is now set as 17.0%.
N enhances resistance to stress corrosion cracking. Furthermore, N is an
austenite former and effectively acts to prevent a .delta.-ferrite phase
from remaining in the annealed condition. Moreover, N controls generation
of permanent strain during the step of deformation, thereby enhancing
shape memory effect. For these effects, at least 0.02% of N is required.
However, if N is included so much, blow holes are generated in an ingot
prepared in the steel making process, and thus, a sound ingot cannot be
obtained. Thus, the upper limit for N is now set as 0.30%.
Co is an austenite former and effectively acts to prevent a
.delta.-ferritic phase from remaining in the annealed condition. Further
Co also effectively serves to control the generation of permanent strain
during the step of deformation and to facilitate formation of a work
induced .epsilon.-phase, thereby enhancing shape memory effect. Moreover,
Co enhances resistance to stress corrosion cracking. For these effects at
least 2.0% of Co must be included. However, even if an increasing amount
of Co is included, the effects are saturated, and so the upper limit for
Co is now set as 10.0%.
Cu is an essential element for the steel according to the invention, since
it remarkably increases resistance to stress corrosion cracking of the
steel. Furthermore, Cu is an austenite former and effectively acts to
prevent a .delta.-ferrite phase from remaining in the annealed condition
thereby to enhance shape memory effect. For these effects more than 0.2%
of Cu is required. However, addition of an unduly excessive amount of Cu
adversely affects hot workability of the steel. Accordingly, the upper
limit for Cu is now set as 3.5%.
Nb, V, Zr and Ti are useful elements to maintain corrosion resistance and
workability of the steel, since they serve to prevent formation of Cr
carbide in the repeated cycle of deformation at not higher than room
temperature and heating at an elevated temperature of 450.degree. C. or
higher. Accordingly, at least one of these elements is preferably included
in an amount of at least 0.05%. However, since these elements are all
ferrite formers, a .delta.-ferrite phase may remain in the annealed
condition, and if these elements are included so much, shape memory effect
is adversely affected, and so the upper limit for the content of each
element is now set as 0.8%.
Mo is effective to enhance corrosion resistance of the steel. However,
since Mo is a ferrite former and if so much Mo is included, a
.delta.-ferrite phase may remain in the annealed condition to decrease
shape memory effect and so the upper limit for Mo is now set as 2.0%.
We have experimentally found that the D value calculated according to the
aforementioned equation is a measure of an amount of a .delta.-ferrite
phase which has remained in the annealed condition and which adversely
affects shape memory effect. We have further found that if the D value is
less than -26.0, so much .delta.-ferrite phase remains to deteriorate
shape memory effect. Accordingly, the alloying components must be mutually
adjusted in order to make the D value not less than -26.0 with their
individual proportions within the aforementioned respective ranges.
The steel according to the invention excellent in resistance to stress
corrosion cracking having the above-described chemical composition may
develop its shape memory function, when treated in the manner as noted
below.
First, the steel is mechanically worked at room or warm temperature to form
an article of a predetermined shape, and the article is annealed to
memorize the shape. The steel according to the invention is substantially
austenitic with no .delta.-ferritic and martensitic phases in the annealed
condition, that is in the condition as annealed and allowed to cool to
room temperature. While by the mechanical working, .epsilon.-phase,
displacement and permanent strain of .alpha.' phase are formed in the
resulting article, the .epsilon.-phase and the permanent strain completely
disappear by annealing the article.
The annealed article is then deformed at a temperature not higher than room
temperature. This deformation at low temperature promote the formation of
a work induced .epsilon.-phase. The shape after the deformation is as such
maintained at temperatures below the As of the steel. When the deformed
article is heated to a temperature of the As point or higher, the original
shape of the article before the deformation is recovered at a high percent
of recovery and maintained even if allowed to cool to room temperature.
The As point of the steels according to the invention is near room
temperature. Accordingly, the heating temperature for recovering the
deformed article to the original shape need not be very high, and may be
at least 100.degree. C., preferably at least 200.degree. C. Since the
transformation of .epsilon.-phase to .delta.-phase at the As point or
higher is accelerated by temperature, the higher the temperature the
shorter the heating time. The heating time may normally be as short as one
minute.
In order to achieve still better shape memory and recovery effect, the
following method is conveniently utilized. First, the steel according to
the invention is mechanically worked at room or warm temperature to form
an article of a predetermined prime shape, and the article is annealed.
Thereafter, the article is deformed or mechanically worked at a
temperature of not higher than room temperature (primary deformation),
heated to a temperature of from 450.degree. C. and 700.degree. C.,and
allowed to cool to room temperature. This primary deformation and heating
may be repeated two or more times. By this treatment a desired primary
shape is achieved and memorized. The article having the primary shape is
deformed to a desired secondary shape at a temperature of not higher than
room temperature (secondary deformation). When the article having the
secondary shape is heated to a temperature of at least the As point of the
steel, the primary shape is recovered and maintained even when allowed to
cool to room temperature. The more the number of the above-mentioned
training cycles comprising the primary deformation at a temperature of not
higher than room temperature and heating at a temperature of from
450.degree. C. to 700.degree. C., the more satisfactorily high percent of
shape recovery can be achieved even when an amount of the secondary
deformation is large. For example, even in a case wherein an amount of the
secondary deformation is as large as 8%, the primary shape can be
recovered at a satisfactorily high percent of recovery. Incidentally, upon
the primary deformation a work induced .epsilon.-phase is formed, and the
lower the deformation temperature the more the amount of an
.epsilon.-phase formed. In the case of a high amount of deformation, a
permanent strain is also generated inevitably. Accordingly, the heating
after the primary deformation must be carried out at a temperature high
enough not only to complete the transformation of the .epsilon.-phase to a
.gamma.-phase but also to remove the permanent strain. For this reason the
heating temperature after the primary deformation should be at least
450.degree. C. However, an unduly high heating temperature is likely to
form Cr carbide which adversely affects corrosion resistance. Accordingly,
the upper limit for the heating temperature is set as 700.degree. C.
After the article has been subjected to the cycle comprising the primary
deformation at a temperature not higher than room temperature and the
heating one or more times, the subsequent secondary deformation at a
temperature of not higher than room temperature only promotes formation of
an .epsilon.-phase with generation of substantially no permanent strain.
Accordingly, if the secondarily deformed article heated to a temperature
of at least the As point of the steel, the primary shape is recovered at a
high percent of recovery even if an amount of the secondary deformation
has been considerably high.
Thus, the invention further provides a method of shape memorizing and shape
recovering of the stainless steel excellent in resistance to stress
corrosion cracking according to the invention or a method of using the
stainless steel according to the invention, which comprises the steps of
processing the stainless steel to an article of a predetermined shape and
annealing the article to memorize the shape, deforming the annealed
article at a temperature of not higher than room temperature, and heating
the deformed article to a temperature of at least 100.degree. C. and
allowing it to cool to room temperature, thereby to recover the memorized
shape.
As a more advantageous method there is provided a method of shape
memorizing and shape recovering of the stainless steel excellent in
resistance to stress corrosion cracking according to the invention, which
comprises the steps of: processing the stainless steel to an article of a
predetermined shape and annealing the article, subjecting the article one
or more times to a training cycle comprising deformation at a temperature
of not higher than room temperature and heating to a temperature of from
450.degree. C. and 700.degree. C., and allowing the so-trained article to
cool to room temperature, thereby to achieve and memorize a primary shape,
deforming the primary shape memorized article to a desired secondary shape
at a temperature of not higher than room temperature, heating it to a
temperature of at least 100.degree. C. and allowing it to cool to room
temperature, thereby to recover the primary shape.
The invention will be further illustrated by the following examples.
EXAMPLES
Each steel melt having a chemical composition (% by weight) indicated in
Table 1 was prepared using a high frequency melting furnace. Steels A1 to
A16 are steels according to the invention, while Steels B1 to B4 are
comparative steels. Steels B1 and B2 have Si and Mn outside the ranges
prescribed herein, respectively. Steel B3 does not contain Cu. Steel B4
has a D value of less than -26.0, although a content of each alloying
element is within the range prescribed herein.
The steel melt was cast into an ingot, forged, hot rolled to a thickness of
3 mm, annealed, cold rolled to a thickness of 2 mm and annealed. From the
cold rolled and annealed sheet a test piece having a width of 10 mm, a
length of 75 mm and a thickness of 2 mm was cut out. This test piece can
be said as a shaped article in the annealed condition. The test piece was
bent at a temperature of -73.degree. C. by 120.degree. with a bend radius
of 8 mm, and set in a constraining apparatus shown in FIG. 1. Under this
constrained condition the test piece was heated to at a temperature of
400.degree. C. for 15 minutes and allowed to cool to room temperature. By
this treatment the test piece tends to recover its original sheet-like
shape under the constrained condition, whereby and a residual stress is
formed in the test piece. The test piece under the constrained condition
was dipped in a boiling 42% MgCl.sub.2 aqueous solution, and a time until
stress corrosion cracking occurred was determined. Results are shown in
Table 2 wherein Mark o indicates that stress corrosion cracking did not
occur within 5 hours whereas Mark x indicates that stress corrosion
cracking occurred within 5 hours.
Shape memory and recovery properties were estimated by the following tests.
The hot rolled sheet having a thickness of 3 mm prepared in the manner
described above, was annealed, and repeatedly cold rolled and annealed to
provide a cold rolled and annealed sheet having a thickness of 1 mm. From
this sheet a test piece having a width of 20 mm, a length of 200 mm and a
thickness of 1.0 mm was cut out. This test piece can be said as a shaped
article in the annealed condition. In one test, the test piece was
deformed at a temperature of 20.degree. C., -73.degree. C. or -196.degree.
C. by imparting a tensile strain of 4%. The deformed piece was heated at a
temperature of 400.degree. C. for 15 minutes and allowed to cool to room
temperature. Percent of shape recovery (Ro) was determined.
In another test, the test piece was deformed at a temperature of 20.degree.
C. or -73.degree. C. by imparting a tensile strain of 6% (primary
deformation), and the deformed piece was heated at a temperature of
600.degree. C. for 15 minutes and allowed to cool to room temperature. The
test piece so treated was again deformed at a temperature of 20.degree. C.
or -73.degree. C. by imparting a tensile strain of 6% (secondary
deformation), and the deformed piece was heated at a temperature of
600.degree. C. for 15 minutes and allowed to cool to room temperature.
Percent of shape recovery (R.sub.T) to the shape after the primary
deformation was determined.
Percent of shape recovery (Ro) was determined in the following manner. An
initial gage length (1.sub.0 =50 mm) was marked on the test piece before
the deformation, and the marked gage length after the tensile strain was
imparted at the low temperature was measured. By subtracting the initial
gage length from the measured gage length, an amount of strain (1.sub.1)
was determined. The gage length after the test piece was heated and
allowed to cool to room temperature was measured, and a length (1.sub.2)
was calculated by subtracting the latter measured gage length from
(1.sub.0 +1.sub.1). Percent of shape recovery was calculated from the
following equation.
R=(1.sub.2 /1.sub.1).times.100(%)
Percent of shape recovery (R.sub.T) was determined in the following manner.
An initial gage length (1.sub.0 =50 mm) was marked on the test piece after
it was primarily deformed, heated and allowed to cool to room temperature
(that is before the secondary deformation), and the marked gage length
after the secondary deformation was measured. By subtracting the initial
gage length from the measured gage length, an amount of strain (1.sub.1)
was determined. The gage length after the secondarily deformed test piece
was heated and allowed to cool to room temperature was measured, and a
length (1.sub.2) was calculated by subtracting the latter measured gage
length from (1.sub.0 +1.sub.1). Percent of shape recovery was calculated
from the equation described above.
The determined Ro and R.sub.T values are also shown in Table 2.
As seen from Table 2, while Comparative steels B1, B2 and B4 are excellent
in resistance to stress corrosion cracking, they have low Ro and R.sub.T
values at 20.degree. C. which indicate unsatisfactory shape memory effect.
They have slightly increased Ro and R.sub.T values at -73.degree. C. and
-196.degree. C., which are still unsatisfactory. Comparative steel B3
containing no Cu is poor in resistance to stress corrosion cracking. In
contrast, Steels A1 to A16 according to the invention are all excellent in
resistance to stress corrosion cracking. They all exhibit excellent shape
memory effect as reflected by their Ro and R.sub.T values at 20.degree. C.
as high as at least 42%, and in particular by their remarkably increased
Ro and R.sub.T values in the case of deformation at lower temperature as
high as at least 65%.
TABLE 1
__________________________________________________________________________
D
Steel
C Si Mn P S Ni Cr N Co Cu Others value
__________________________________________________________________________
A A1 0.035
4.86
9.88
0.023
0.003
3.98
11.94
0.032
6.01
1.15
-- -21.4
A2 0.038
5.27
10.81
0.024
0.005
3.02
12.12
0.043
5.88
2.49
-- -22.0
A3 0.041
5.16
12.51
0.024
0.003
1.98
12.35
0.045
6.15
3.08
-- -21.6
A4 0.036
4.72
14.92
0.025
0.003
1.92
12.18
0.039
6.02
2.11
-- -20.8
A5 0.042
4.78
17.87
0.024
0.004
0.06
12.01
0.040
6.10
1.70
-- -21.7
A6 0.041
4.75
17.49
0.024
0.004
0.08
12.23
0.045
7.05
0.53
-- -22.5
A7 0.029
4.80
21.58
0.025
0.004
0.87
12.51
0.186
3.54
1.21
-- -21.0
A8 0.078
3.75
15.22
0.025
0.004
1.21
15.17
0.041
8.99
1.52
-- -20.1
A9 0.009
4.70
14.88
0.024
0.003
6.01
11.88
0.102
6.82
1.14
-- -16.9
A10
0.010
4.78
17.52
0.025
0.004
4.13
12.28
0.081
8.05
0.98 -18.5
A11
0.037
4.67
12.27
0.024
0.004
4.01
12.16
0.041
6.50
1.06
Nb:0.27 -20.7
A12
0.040
4.70
17.86
0.025
0.004
1.02
12.51
0.039
6.21
1.89
V:0.32 -22.1
A13
0.039
4.66
12.52
0.025
0.004
3.95
12.11
0.041
6.55
0.95
Nb:0.24, Zr:0.19
-21.0
A14
0.040
4.78
12.98
0.025
0.005
3.98
12.26
0.045
6.32
0.98
V:0.25, Ti:0.11
-21.2
A15
0.009
4.84
15.52
0.024
0.004
2.01
12.23
0.088
6.50
2.02
Nb:0.31 -22.4
A16
0.040
4.88
17.76
0.025
0.004
4.02
12.17
0.040
6.05
1.35
Nb:0.27, Cu:1.24
-21.9
B B1 0.040
1.53
18.07
0.024
0.005
2.02
13.90
0.045
7.02
1.60
-- -13.0
B2 0.035
5.18
4.05
0.025
0.004
2.12
13.05
0.040
6.25
1.52
-- -27.3
B3 0.042
5.13
14.93
0.025
0.004
3.92
12.18
0.039
6.02
-- -- -21.8
B4 0.036
5.23
8.92
0.025
0.003
1.98
13.52
0.031
3.88
1.06
-- -29.3
__________________________________________________________________________
A: Steel according to the invention
B: Comparative steel
TABLE 2
__________________________________________________________________________
Stress Corrosion
Cracking R.sub.o value (%)
R.sub.T value (%)
Steel Resistance*.sup.1
20.degree. C.
-73.degree. C.
-196.degree. C.
20.degree. C.
-73.degree. C.
__________________________________________________________________________
A A1 .smallcircle.
56 82 91 52 80
A2 .smallcircle.
52 79 86 50 78
A3 .smallcircle.
54 81 90 52 78
A4 .smallcircle.
50 77 85 48 77
A5 .smallcircle.
51 80 88 48 78
A6 .smallcircle.
51 78 87 50 78
A7 .smallcircle.
49 79 86 47 76
A8 .smallcircle.
45 72 82 42 65
A9 .smallcircle.
52 79 88 51 80
A10 .smallcircle.
50 80 89 48 79
A11 .smallcircle.
53 82 90 52 81
A12 .smallcircle.
54 83 89 50 80
A13 .smallcircle.
52 80 88 50 78
A14 .smallcircle.
50 79 86 49 77
A15 .smallcircle.
55 82 90 50 78
A16 .smallcircle.
53 82 91 51 79
B B1 .smallcircle.
10 16 17 9 13
B2 .smallcircle.
19 25 28 15 22
B3 x 51 79 87 48 78
B4 .smallcircle.
25 44 47 20 29
__________________________________________________________________________
A: Steel according to the invention
B: Comparative steel
*.sup.1 Stress Corrosion Cracking Resistance
.smallcircle. (Time until break > 5 hrs.)
x (Time until break .ltoreq. 5 hrs.)
As demonstrated herein, the stainless steel according to the invention
develop excellent shape memory effect by subjecting to deformation at low
temperature or to repetition of deformation at low temperature and heating
at a temperature of from 450.degree. C. to 700.degree. C., in spite of the
fact that it contain more than 10% of Cr to enhance corrosion resistance.
Furthermore, it is excellent in resistance to stress corrosion cracking.
Accordingly, the steel according to the invention are particularly useful
as a material for fixing or fastening parts of machines, or a pipe joint
in the fields where corrosion resistance and in particular resistance
stress corrosion cracking is required.
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